This invention relates to tractor and implement hydraulic lift systems. A current draft control system for semi-mounted and trailed implements comprises an implement mouted remote cylinder that is connected in series with a tractor mounted hydraulic power lift cylinder for raising and lowering the tractor implement hitch. The coaction in the system is such that both cylinders extend and retract synchronously, raising and lowering both ends of the implement, front and rear, simultaneously.
Prior art patents of interest relative to the present invention are C. E. McKeon et al, U.S. Pat. No. 3,575,241, issued Apr. 20, 1971, for a "Tractor Hydraulic Lift Control System," and R. W. Wilson, U.S. Pat. No. 3,753,467, issued Aug. 21, 1973, for a "Tractor Implement Hydraulic Lift System."
The McKeon patent is directed to a tractor hydraulic lift system in which the height of the implement supporting hitch may be controlled responsive to driveline torque, a soil engaging implement mounted on the hitch being raised or lowered to maintain tractor driveline torque within selected predetermined limits. The Wilson patent carries the system a step further in that it discloses a system in which the hydraulic lift control of the tractor is utilized to regulate the height of a semi-mounted or trailed implement by regulating flow to and from a hydraulic cylinder mounted on the implement, thus also controlling the height of the implement responsive to tractor driveline torque.
When the operator of the tractor desires to use the draft control system, such as disclosed by Wilson, he must first engage an auxiliary services valve which directs pump oil to the remote cylinder. Return oil from the remote cylinder is directed to the hydraulic power lift cylinder, hence raising the implement by simultaneously extending both cylinders.
There are three major concerns with the known system which are detrimental to the farmer and the manufacturer of the equipment. First, the known system requires a complex sequence in which the controls must be operated to phase the remote cylinder with the hydraulic power lift cylinder for the initial and operational positioning of the earth working implement. Due to leakage of oil in either or both cylinders, the cylinders can become operationally out of phase. Second, the known system requires a unique remote cylinder which must be required to the customer with the tractor. This is because the remote cylinder in this system must match the volume of the hydraulic power lift cylinder. This unique remote cylinder contains a valve in the piston which opens at the end of the cylinder stroke to allow oil transfer to the hydraulic power lift cylinder. Third, the known system is susceptible to accidental overheating of the tractor hydraulic system in the event the operator disconnects the remote cylinder and forgets to shift the auxiliary service control valve to a position in which it continues to pump oil for the now non-existant remote cylinder, thus building up internal pressures in the tractor hydraulic system.
The present invention has as its objective a system that permits the use of any standard remote cylinder and automates the phasing and auxiliary service control valve functions.